1. Technical Field of the Invention
The present invention relates generally to spark plugs for use in internal combustion engines of automobiles and cogeneration systems.
More particularly, the invention relates to a spark plug with an improved structure that ensures a high capability of the spark plug to ignite the air-fuel mixture (referred to as ignition capability of the spark plug hereinafter).
2. Description of the Related Art
Conventional spark plugs for use in internal combustion engines generally include a tubular metal shell, an insulator, a center electrode, and a ground electrode.
The tubular metal shell has a threaded portion for fitting the spark plug into a combustion chamber of the engine.
The insulator has a center bore formed therethrough; it is fixed in the metal shell such that an end thereof protrudes from an end of the metal shell.
The center electrode is secured in the center bore of the insulator and has an end that protrudes from the end of the insulator.
The ground electrode has a base end joined to the end of the metal shell and a tip portion that faces the end of the center electrode in the axial direction of the insulator through a spark gap.
In recent years, the demand for higher power output and improved fuel economy of internal combustion engines has required increasing the sizes of intake and exhaust valves for the engine and securing a water jacket for cooling of the engine. This results in a decreased space available for installing a spark plug in the engine, thus requiring the spark plug to have a compact (more specifically, slenderized) structure.
Specifically, the threaded portion of the metal shell in a spark plug had a size of M14 as specified in JIS (Japanese Industrial Standards) in the past; however, the threaded portion is now required to have a size less than or equal to M12 as specified in JIS.
In such a compact spark plug 9, as shown in FIG. 13, an insulation distance, which is the minimum distance between the insulator 92 and a base end portion 951 of the ground electrode 95, is accordingly reduced.
Consequently, when carbon has deposited on the surface of the insulator 92, instead of normal sparks to be discharged across the spark gap G, “side sparks” P can be discharged. The side sparks P, here, denote sparks which creep from the center electrode 93 along the outer surface of the insulator 92, and jump to the base end portion 951 of the ground electrode 95.
More specifically, in the case that the surface of the insulator 92 has been fouled with carbon that is electrically conductive, the electrical potential on the outer surface of the insulator 92 increases when an electrical voltage is applied between the center electrode 93 and the ground electrode 95. Then, when the electrical potential has increased above a certain level, the side sparks P can be discharged across the air gap between the outer surface of the insulator 92 and the base end portion 951 of the ground electrode 95.
Unlike normal sparks, the side sparks P cannot reliably ignite the air-fuel mixture in the combustion chamber of the engine, thus decreasing the ignition capability of the spark plug.
Accordingly, side sparks have become a great obstacle to development of compact spark plugs.
To prevent side sparks from occurring, Japanese Patent First Publication No. S60-235379 discloses a spark plug that has properly specified dimensional parameters such as the end diameter of the insulator.
However, in the disclosed spark plug, the insulator has a long portion that is to protrude into the combustion chamber of the engine. Consequently, though side sparks can be prevented from occurring, it is still difficult to secure a high ignition capability of the spark plug.
More specifically, since the portion of the insulator protruding into the combustion chamber is made long, the length from the end of the insulator to the area where the insulator is connected to the metal shell through a metal ring is accordingly long.
As a result, the temperature at the end of the insulator will be high, so that it becomes difficult for carbon to deposit on the surface of the insulator, thus preventing side sparks from occurring.
However, at the same time, the high temperature at the end of the insulator may cause a pre-ignition of the air-fuel mixture, thus decreasing the ignition capability of the spark plug.
Moreover, since the portion of the insulator protruding into the combustion chamber is made long, the space S between the insulator and the ground electrode as shown in FIG. 13 is accordingly small.
As a result, it becomes difficult for the initial flame to propagate, thus decreasing the ignition capability of the spark plug.
Accordingly, it is desired to prevent side sparks from occurring without making the portion of the insulator protruding into the combustion chamber long.